Repetition is often a principal part of composing music and is a natural consequence of the fact that distinct instruments, voices, and tones are used to create a soundtrack. Thus, similar blocks of music are likely to be found within a single musical piece, an album of songs from a single author, or within instrument solos. Finding similarities within musical pieces is a challenging task. First, redundancies are often superimposed. Next, it is difficult to model redundancies in music as opposed to, for example, video where camera and object motion have well-defined mathematical models.
In particular, music that is performed by humans commonly exhibits like patterns that sound repetitive to the human ear but have significant variance in amplitude, pitch, and timing, mathematically. Detecting similar blocks in such humanly performed pieces is a complex task that is not addressed by modem psychoacoustic models. Perceptually similar blocks of music can be quite distant in a Euclidean sense. However, the human auditory system may perceive them as equivalent.
On the other hand, electronic music is often generated using exact replication and/or mixing of certain blocks of audio. In a memory-based model of the source signal, this phenomenon can be utilized to improve compression rates achieved by conventional memory-less or simple prediction-based audio compression algorithms such as MP3 or AAC (see, for example, K. Brandenburg, “MP3 and AAC explained,” Proceedings of the AES 17th International Conference on High Quality Audio Coding, 1999). An overview of key techniques for perceptual coding of audio can be found in T. Painter and A. Spanias, “Perceptual coding of digital audio,” Proceedings of the IEEE, pp.451-513, April 2000.
Kirovski and Petitcolas use naturally occurring similarities in music to perform test attacks on watermark-based content protection systems (D. Kirovski and F. A. P. Petitcolas, “Replacement attack on arbitrary watermarking systems,” ACM Workshop on Digital Rights Management, 2002. A similar approach is needed to achieve better compression rates for music.